The term "compact" designates a sintered product consisting of grains bonded to each other by bridges created by diffusion of matter in the plastic state, also called bridging. This sintering in the plastic state is obtained at pressures and temperatures on the order of the size of pressures and temperatures used for the synthesis of diamond grains.
The term "compact" does not cover abrasives comprising a support of silicon carbide and polycrystalline diamond, nonsintered because it is not subjected during production to temperatures and pressures that are sufficient to make possible the intergrowth of diamond grains; in these products, the gaps between grains of the composite are occupied by a compound of silicon and a metal such as nickel, as shown in U.S. Pat. No. 4,241,135. These products exhibit a poor resistance to abrasion because of the absence of sintering.
Nor does the term "compact" include composite abrasives as shown in U.S. Pat. No. 4,124,401, comprising a polycrystalline diamond compound cemented by a binder containing silicon associated with a carbide support whose cohesion is provided by cobalt. The absence of a catalyst and of sintering during the production of the diamond compound prevents the formation of direct bridges between the diamond grains. A sintered compact having a highly rigid skeleton is not obtained, but rather a product that can be qualified as being cemented by a binder. Such a product is sometimes called "cemented," according to terminology derived from English.
In some compacts of the type defined above, as shown in U.S. Pat. No. 3,239,921, obtained at a temperature able to exceed 1,750.degree., the gaps in the compact are occupied by a conversion catalyst such as Co, Va, Ti, Zr, Cr, Si. These products have the drawback of rapidly degrading (poor resistance to abrasion) because the sintering is performed in the absence of a sufficient quantity of diamond grains.
Products are known in which a compact is directly bonded to a metal carbide support (generally tungsten carbide). French patent 2,089,415 describes such a composite product consisting of a diamond compact on a tungsten carbide support; the compact and the carbide contain the same additive which can be cobalt, nickel or iron, this additive assuming, on the one hand, the role of diamond solvent-catalyst and, on the other hand, the role of binder for carbide sintering. These products exhibit the drawback of rapidly degrading when the active part is brought to a temperature exceeding about 700.degree. because, on the one hand, of stresses induced in the metal matrix as a result of thermal expansion of this matrix and, on the other hand, because of the tendency of the diamond in contact with the catalyst to revert to the graphite state when it is brought to a high temperature without simultaneously being subjected to a high pressure. This graphitization affects the structural integrity of the composite.
Such a product with a cobalt binder is available on the market and currently used thus cannot be used for work which brings it to temperatures higher than 750.degree..
The use of a nickel binder would provide a partial solution to these problems, but the mechanical properties of tungsten carbide comprising such an additive are very inferior to those of tungsten carbide with cobalt binder, which explains why these products up to now have not had industrial applications.
Diamond abrasives were recently proposed in Japanese 164073 and European 0 198 653 that are not associated with a tungsten carbide support and are produced by direct sintering of diamond grains in the presence of binder containing nickel, for example nickel alloyed with chromium.
These products exhibit the drawback of not being able to be brazed on tools, which very seriously limits their applications and thus their use.
Therefore, no composite abrasive, i.e., consisting of a compact and a support solid with it, exists that provides simultaneously the qualities of thermostability and resistance to abrasion desired for current abrasives.